Escalating demands for high density and performance associated with ultra large scale integration require semiconductor devices with design features of 0.25 microns and under, e.g. 0.18 microns, increased transistor and circuit speeds, high reliability, and increased manufacturing throughput. The reduction of design features to 0.25 microns and under challenges the limitations of conventional interconnection technology, including conventional photolithographic, etching, and deposition techniques.
Conventional methodology for forming patterned metal layers employs a subtractive etching or etch back step as the primary metal patterning technique. Such a method involves the formation of a first dielectric layer on a semiconductor substrate, typically monocrystalline silicon, with conductive contacts formed therein for electrical connection with an active region on the semiconductor substrate, such as a source/drain region. A metal layer, such as aluminum or an aluminum alloy, is deposited on the first dielectric layer, and a photoresist mask is formed on the metal layer having a pattern corresponding to a desired conductive pattern. The metal layer is then etched through the photoresist mask to form the conductive pattern comprising metal features separated by gaps, such as a plurality of metal lines with interwiring spacings therebetween.
A second dielectric layer is then applied to fill in the gaps. For example, a dielectric material, such as spin on glass (SOG), is deposited to fill in the gaps between the metal features, and baked at a temperature of about 300.degree. C. to about 350.degree. C., and then cured in a vertical furnace at about 350.degree. C. to about 400.degree. C. for a period of time up to about one hour, depending upon the particular SOG material employed, to effect planarization. Another oxide is deposited by plasma enhanced chemical vapor deposition PEGMD), and the surface is planarized, for example, by conventional etching or chemical-mechanical polishing (CMP) planarization techniques.
As feature sizes, e.g., metal lines and interwiring spacings, shrink to 0.25 microns and below, such as 0.18 microns, it becomes increasingly difficult to satisfactorily fill in the interwiring spacings voidlessly and obtain adequate step coverage. It also becomes increasingly difficult to form a reliable interconnection structure. A through-hole is typically formed in a dielectric layer to expose an underlying metal feature, wherein the metal feature serves as a landing pad for the through-hole. Upon filling the through-hole with conductive material, such as a metal plug to form a conductive via, the bottom surface of the conductive via is in contact with the metal feature.
A conventional conductive via is illustrated in FIG. 1, wherein a first metal feature 100 of a first patterned metal layer is formed on first dielectric layer 110 and exposed by a through-hole 120 etched in a second dielectric layer 130. The first metal feature 100, which has side surfaces that taper outwardly somewhat due to etching, typically has a composite structure comprising a lower metal layer 102, e.g., titanium (Ti) or tungsten (W), an intermediate or primary conductive layer 104, e.g., aluminum (Al) or an Al alloy, and an anti-reflective coating (ARC) 106, such as titanium nitride (TiN).
Gaps between the first metal feature 100 and another metal feature 190 of the first patterned metal layer are filled with dielectric material 180, such as SOG or HDP oxide. A dielectric layer 130, such as silicon dioxide derived from tetraethyl orthosilicate (TEOS) or silane, is then former upon the first patterned metal layer, as by PECVD, and planarized, as by CMP.
In accordance with conventional practices, through-hole 120 is formed so that first metal feature 100 completely surrounds the bottom opening, thereby serving as a landing pad for the metal plug filling the through-hole 120 to form the conductive via 160. The conductive via 160 electrically connects the first metal feature 100 and a second metal feature 140, which is part of a second patterned metal layer.
The second metal feature 140 is also typically a composite structure comprising a lower metal layer 142, a primary conductive layer 144, and an ARC 146. The plug filling the through-hole 120 to form the conductive via 160 is typically formed as a composite comprising a first adhesion promoting layer 150, which is typically a refractory material, such as TiN, Ti--W, or Ti--TiN, and a primary plug filling metal 170 such as W. Metal features 100 and 140 typically comprise metal lines with interwiring spacings therebetween conventionally filled with dielectric material 180, such as SOG or HDP oxide.
The reduction of design features to the range of 0.25 microns and under requires extremely high densification. The conventional practice of forming a landing pad enclosing the bottom surface of a conductive via utilizes a significant amount of precious real estate on a semiconductor chip which is antithetic to escalating high densification requirements. In addition, it is extremely difficult to voidlessly fill through-holes having such reduced dimensions because of the extremely high aspect ratio, i.e., height to width of the through-hole. Accordingly, conventional remedial techniques comprise purposely widening the through-hole to decrease the aspect ratio. As a result, misalignment may occur wherein the bottom surface of the conductive via is not completely enclosed by the underlying metal feature. This type of via is called a "borderless via", which also conserves chip real estate.
The use of borderless vias, however, creates new problems. For example, as a result of misalignment, the SOG gap filling layer is penetrated by etching when forming a misaligned through-hole, due to the low density and poor stability of SOG. As a result of such penetration, moisture and gas accumulate, thereby increasing the resistance of the interconnection. Moreover, spiking can occur, i.e., penetration of the metal plug to the substrate causing a short. Referring to FIG. 2, first dielectric layer 210 is formed on a substrate (not shown) and a first metal pattern comprising a first metal feature 200, e.g. a metal line, comprising anti-reflective coating 206, is formed on first dielectric layer 210. Gaps, such as the gap between the first metal feature 200 and another metal feature 290, are filled with SOG 280. Dielectric layer 230 is then deposited and a misaligned through-hole 220 formed therein exposing a portion of the upper surface and at least a portion of a side surface of first metal feature 200, and penetrating into and exposing a portion of SOG layer 280. Upon filling the through-hole 220 with a metallic plug 260, typically comprising an initial barrier layer 250 and tungsten 270, a spike 222 is formed.
Alternatively, SOG may be replaced by hydrogen silsesquioxane (HSQ), which offers several advantages. HSQ is relatively carbon free, thereby avoiding poison via problems and the need to etch back HSQ below the upper surface of the metal lines to avoid shorting. In addition, HSQ exhibits excellent planarity and is capable of gap filling interwiring spacings less than 0.15 microns employing conventional spin-on equipment. Moreover, HSQ has a relatively low dielectric constant, between 3.0 and 4.0 at 1 Mhz, compared to about 4.9 for plasma deposited SiO.sub.2. A lower dielectric constant reduces parasitic, interline capacitive coupling, thereby increasing circuit speed.
HSQ, however, is susceptible to degradation during processing leading to various problems, including an increase in its dielectric constant. HSQ typically contains between about 70% and about 90% Si--H bonds. Upon exposure to an O.sub.2 -containing plasma when stripping tang the photoresist mask for defining a misaligned or borderless via, however, a considerable number of Si--H bonds are broken and Si--OH bonds are formed. In fact, upon treatment with an O.sub.2 -containing plasma, as little as about 20% to about 30% of the Si--H bonds may remain in the deposited HSQ film. It has been found that breaking the Si--H bonds results in a local increase in the dielectric constant and, consequently, the amount of intraline capacitive coupling.
Due to the problems attendant upon forming borderless vias, it is desirable to reduce the occurrence of through-hole misalignment and, hence, the number of borderless vias. If the metal feature is widened to increase the landing area for the via, misalignments can be avoided. However, there are disadvantages attendant upon widening the metal features. A conventional metal structure typically has a profile characterized by outwardly tapering side surfaces, wherein the top surface has a width smaller than the bottom surface, as illustrated in FIG. 1. For example, first metal feature 100, the top surface comprising anti-reflective coating 106 is located, can have a width of about 0.45 .mu.m, while the bottom surface has a width of about 0.55 .mu.m. Outwardly tapering metal structures are considered desirable to facilitate voidless gap filling, as with HDP oxide.
Increasing the width of the metal features leads to such problems as bridging, increased intraline capacitive coupling, and altered electrical dimensions. Bridging is characterized by a connection across the gap between metal structures, e.g. first metal feature 100 and a metal feature 190, and is due to incomplete etching down to the first dielectric layer 110, In this situation, an electrical connection, or a "short," may undesirably occur along the unetched metal remaining at the bottom of the gap. Generally, the smaller the gap, the greater the risk of bridging.
Intraline capacitive coupling is inversely proportional to the distance between the metal structures. Widening metal features reduces the distance between the metal features, thereby increasing the intraline capacitive coupling. Consequently, widening the metal lines and other metal features results in slower circuit speed due to the increased intraline capacitive coupling.
Another disadvantage attendant upon widening metal features is that the electrical dimensions are changed, e.g. the resistance of the lines. A changed electrical dimension may necessitate redesigning the metallization layers, thereby undesirably increasing the costs and postponing chic development.